Electrochemical capacitor

ABSTRACT

An electrochemical capacitor comprising a pair of oxidation-reduction active electrodes and an electrolyte solution or an ion-conductive solid electrolyte between the electrodes, wherein the electrodes contain a specific copolymer, thereby providing the electrochemical capacitor high in energy density and excellent in heat-resistance and charge/discharge cycle features.

TECHNICAL FIELD

The present invention relates to an electrochemical capacitor used in avariety of electronic devices.

BACKGROUND ART

An electric double layer capacitor characterized by having a capacitancevalue between that of a battery and that of a capacitor has been widelyused as a DC power source of a small wattage rating to replace a back-uppower supply or to serve as an auxilary/alternate cell of a secondarybattery for ICs and memories. However, the energy density of electricdouble layer capacitors available at present is some tenth of the energydensity of such secondary batteries as a lead acid storage battery, anickel hydrogen secondary battery and therefore a much higher energydensity has been required of electric double layer capacitors.

On the other hand, the advantages in using an electric double layercapacitor are as follows:

Such environmentally burdening material as lead, cadmium and the likeare used in the electrodes of general purpose secondary batteries,whereas electric double layer capacitors use electrodes mainly formed ofsuch carbon materials as activated carbon and the like that are safe andless burdensome on the environment. In addition, electric double layercapacitors utilize electric double layer capacitance created on theinterface between an electrode and an electrolyte, not relying on theconversion of a chemical reaction to electric energy like in a storagebattery, resulting in a high degree of efficiency at the time ofcharging/discharging and also a long cycle life.

An electric double layer capacitor comprises a pair of polarizableelectrodes, each of which is formed by the steps of applying a slurrycomprised of activated carbon, carbon black, a binder and the like ontoan electric current collector formed of an aluminum foil and the like toa uniform thickness and finally drying the slurry, disposed opposite toeach other with a porous separator, which is impregnated with acorrosion-resistant electrolyte, sandwiched between the polarizableelectrodes.

A water-soluble type electrolyte or an organic solvent type electrolyte(a nonaqueous electrolyte) is used as the electrolyte for electricdouble layer capacitors. However, the withstand voltage of awater-soluble type electrolyte is low (about 1.2 V), resulting in aproblem of difficulties to realize electric double layer capacitors witha high energy density. On the other hand, the withstand voltage of anonaqueous electrolyte ranges from 2.3 V to 3.0 V but the electricconductivity thereof is low when compared with a water-soluble typeelectrolyte, resulting in a problem of increasing the internalresistance of electric double layer capacitors. In recent years, variousstudies have been carried out to solve these problems and suchinventions as disclosed in the Japanese Patent Application UnexaminedPublication Nos. S61-32509 and S62-237715, the International PatentApplication Unexamined Publication No. WO96/20504 and the like have beenmade.

The activated carbon to form electrodes for electric double layercapacitors is prepared by carbonizing at high temperatures such rawmaterials as coconut husk, sawdust, phenolic resin, petroleum coke, coalcoke and the like and thereafter by putting through such treatments assteam-activation, zinc chloride activation, alkali activation and thelike. It is generally said that the surface area per unit weight of theactivated carbon for electric double layer capacitors is the larger, thehigher capacitance per unit weight is allowed to be gained. However, anincrease in the surface area per unit weight of the activated carbonresults in a decrease in the bulk density thereof, thereby causing aproblem of decreasing the energy density per unit volume of electricdouble layer capacitors.

Further, in order to solve the foregoing problems associated withelectric double layer capacitors, such new devices, which have theenergy density comparable with that of secondary batteries such as alead acid storage battery and the like, as an electrochemical capacitoras described in the Japanese Patent Application Unexamined PublicationNo. H6-104141 and the like have been recently developed and many reportscover such developments. In addition, a π conjugate polymer as describedin Chem. Rev. 1997, 97, 207-281 and studied for use in the electrode ofsecondary batteries is also allowed to be used in an electrochemicalcapacitor theoretically.

However, even among the foregoing conventional electrochemicalcapacitors, an electrochemical capacitor with an electrode formed ofruthenium oxide or a composite of ruthenium oxide and other metal oxidesrealizes a high energy density and excellent cycle characteristics onone hand but on the other hand there is such a problem as the rutheniumused in the electrode being very expensive and the like. Anelectrochemical capacitor with such conductive polymers having a πconjugate as polypyrrole, polythiophene, polyaniline and the derivativesthereof and the like used as the electrode materials is allowed torealize a high energy density but also has a problem of being inferiorto the electric double layer capacitors utilizing activated carbon interms of heat resistance and cycle characteristics. This is attributedto the heat resistance of a π conjugate type conductive polymer and anirreversible chemical reaction occurring between the electrolytematerial needed for an electrochemical capacitor and the π conjugatetype conductive polymer. Therefore, an electrode material, which hashigh heat resistance and does not cause an irreversible chemicalreaction with the electrolyte material, is being looked for.Furthermore, it is generally difficult for a π conjugate conductivepolymer to be resolved/melted, resulting in a problem of not allowingthe electrode for electrochemical capacitors to be produced readily.

The present invention deals with the foregoing problems that have beenso far existing and aims at providing an electrochemical capacitorhaving a high energy density and excelling in heat resistance andcharge/discharge cycle characteristics.

SUMMARY OF THE INVENTION

In order to solve the foregoing problems, the present inventiondiscloses an electrochemical capacitor comprising a pair ofoxidation-reduction active electrodes and an electrolyte or an ionicconductive solid electrolyte sandwiched between the oxidation-reductionactive electrodes, in which the oxidation-reduction active electrodecontains a copolymer that has a chemical formula 1 as described below,thereby allowing an electrochemical capacitor having a high energydensity and excelling in heat resistance and charge/discharge cyclecharacteristics to be realized.

R^(a)—(C₂R^(c) ₄)_(l)—(OC₂R^(d) ₄)_(m)—(X)_(n)—R^(b), or

R^(a)—[—(C₂R^(c) ₄)_(l)—(OC₂R^(d) ₄)_(m)—(X)_(n)—]_(p)—R^(b)  (ChemicalFormula 1)

, where R^(a) and R^(b) are, respectively, hydrogen, fluorine, an alkylgroup with 1 to 15 of carbon, a fluoroalkyl group, an alkenyl group anda fluoroalkenyl group, or a phenyl group, a fluorophenyl group, asulfone group, an amino group, a nitro group and a cyano group, andR^(a) and R^(b) are allowed to be the same with or different from eachother.

Also, R^(c) and R^(d) are selected from hydrogen, fluorine, an alkylgroup with 1 to 15 of carbon, a fluoroalkyl group, an alkenyl group anda fluoroalkenyl group, or a phenyl group, a fluorophenyl group, asulfone group, an amino group, a nitro group and cyano group, or achemical formula A, and the respective four R^(c) and R^(d) are allowedto be the same with or different from one another.

The chemical formula A is described by a general expression as follows:

—(CO)OR^(e), —(CH₂)R^(a)—(OC₂R^(b) ₄)_(b)—OR^(e),

where R^(e) is hydrogen or an alkyl group, and “a” and “b” are integersof 0 to 20 and allowed to be the same with or different from each other.

In the chemical formula 1, “l” and “m” are integers of 0 or larger andnot 0 at the same time, “n” and “p” are natural numbers of 1 or larger.X is any one species, or two or more species out of the chemical speciesdescribed by a general expression as follows:

R1 to R6 are, respectively, hydrogen, fluorine, an alkyl group with 1 to20 of carbon, which is allowed to be replaced with a hydroxyl group, afluoroalkyl group, an alkenyl group and a fluoroalkenyl group, or aphenyl group, a fluorophenyl group, a sulfon group, an amino group, anitro group and a cyano group with part or all thereof being allowed tobe linked with one another to form rings. Also, R1 to R6 are allowed tobe the same with or different from one another.

PREFERRED EMBODIMENTS OF THE INVENTION

An electrochemical capacitor in a first exemplary embodiment of thepresent invention comprises a pair of oxidation-reduction activeelectrodes with an electrolyte or an ionic conductive solid electrolytesandwiched between the oxidation-reduction active electrodes, in whichthe oxidation-reduction active electrode contains a copolymer expressedby the chemical formula 1. According to the foregoing structure, thecopolymerization taking place between a conductive polymer and a polymerwith high heat resistance and a powerful affinity for solvent on theelectrodes of the electrochemical capacitor allows the electrodematerial to enhance the heat resistance and cycle characteristicsthereof, thereby achieving the effects of a higher energy density incomparison with a prior art electric double layer capacitor employingelectrodes formed of activated carbon and also enhanced heat resistanceand cycle characteristics when compared with an electrochemicalcapacitor using electrodes formed of a conductive polymer.

An electrochemical capacitor in a second exemplary embodiment of thepresent invention is the same as in the first exemplary embodimentexcept for X in the chemical formula 1 being 3-(4-fluorophenyl)thiophene. The introduction of an electron-withdrawing 4-fluorophenylgroup in thiopene realized by the 3-(4-fluorophenyl) thiophene makes iteasy for doping and de-doping of electrolyte by cation to be carriedout, thereby achieving the effects of realizing an electrochemicalcapacitor of high capacitance and also excellent cycle characteristics,in particular.

An electrochemical capacitor in a third exemplary embodiment of thepresent invention is the same as in the first exemplary embodimentexcept for X in the chemical formula 1 being 3-sulfonylthiophene.According to the foregoing structure, the sulfonyl group of the3-sulfonylthiophene allows a conductive polymer to be self-doped,thereby particularly reducing the change in electric conductivity of theconductive polymer and achieving the effects of realizing anelectrochemical capacitor with a low internal resistance value.

An electrochemical capacitor in a fourth exemplary embodiment of thepresent invention is the same as in the first exemplary embodimentexcept for X in the chemical formula 1 being 3,4-disulfonylthiophene.According to the foregoing structure, the sulfonyl group of the3,4-disulfonylthiophene allows a conductive polymer to be self-doped,thereby particularly reducing the change in electric conductivity of theconductive polymer, further making it easy for the electrolyte to beimpregnated in the polymer because of the steric hindrance of twosulfonyl groups and achieving the effects of realizing anelectrochemical capacitor with a large capacitance value and also a lowinternal resistance value.

An electrochemical capacitor in a fifth exemplary embodiment of thepresent invention is the same as in the first exemplary embodimentexcept for X in the chemical formula 1 being ethylenedioxithiophene.According to the foregoing structure, the stereoregularity of aconductive polymer is enhanced and the conductive polymer showsexcellent electrical conductivity, thereby allowing an electrode formedof ethylenedioxithiophene to be particularly low in resistance andachieving the effects of realizing an electrochemical capacitor with asmall internal resistance value.

An electrochemical capacitor in a sixth exemplary embodiment of thepresent invention is the same as in the first exemplary embodimentexcept for the oxidation-reduction electrode to be containing at leastone or more of oxide of ruthenium, manganese, vanadium, titanium,aluminum, nickel, iron, magnesium and silicon. According to theforegoing structure, a mixture of the oxides and a copolymer of thepresent invention leads to an increased degree of dispersion of thecopolymer in the oxidation-reduction electrode, thereby achieving theeffects of realizing an electrochemical capacitor with a largecapacitance value and also a low internal resistance value.

Next, a detailed description is given to the exemplary embodiments ofthe present invention.

As a polar aprotic solvent used in the present invention, there are suchcyclic carbonic esters as ethylene carbonate, propylene carbonate,butylene carbonate, vinylene carbonate and the like, such chain carbonicesters as dimethyl carbonate, methylethyl carbonate, diethyl carbonate,methylpropyl carbonate, methylisopropyl carbonate and the like, suchcyclic esters as γ-buthyrolactone, γ-valerolactone,3-methyl-γ-buthyrolactone, 2-methyl-γ-buthyrolactone and the like, suchchain esters as methyl formate, ethyl formate, methyl acetate, ethylacetate, propyl acetate, methyl propionate, methyl butyrate, methylvalerate and the like, such cyclic ethers as 1,4-dioxan, 1,3-dioxolane,tetrahydrofuran, 2-methyltetrahydrofuran, 2-methyl-1,3-dioxolane and thelike, such chain ethers as 1,2-dimeth-oxyethane, 1,2-diethoxyethane,diethyl ether, dimethylether, methylethylether, dipropylether and thelike, such sulfur complex compounds as sulfolane and the like and suchnitrites as acetonitrile, benzonitrile, butyronitrile and the like.

Also, as the foregoing ester carbonate, such a cyclic ester carbonate aspossessing a halogene substitution alkyl as described in the JapanesePatent Application Unexamined Publication No. H9-63644 is allowed to beused in addition to the cyclic ester carbonates enumerated in above. Asthe cyclic ester carbonates as described above, there are providedmonofluoromethylethylene carbonate, difluoromethylethylene carbonate,trifluoromethylethylene carbonate and the like.

In addition, these solvents are allowed to be used alone or by havingtwo or more thereof mixed.

As the electrolyte cation used in the present invention, such quaternaryonium ions as tetrabutylammonium, tetraethylammonium,triethylmonomethylammonium, tetrabutylphosphonium, tetraethylphosphoniumand the like or the electrolyte cation and the like as described in theInternational Patent Application Unexamined Publication No. WO95/15572are allowed to be used.

As the electrolyte anion used in the present invention, such as ClO₄ ⁻,BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, SbF₆ ⁻, perfluoroalkanesulfonic acid ion with twoor more of carbon, perchloroalkanesulfonic acid ion,perfluoroalkanecarboxylic acid ion, perchloroalkanecarboxylic acid ion,bis (perfluoroalkyl) sulfonylimide ion, tris (perfluoroalkyl)sulfonylmethid ion and the like are used.

As the polymer for gel electrolyte, such as polyethylene oxide,polypropylene oxide, polyvinylidene fluoride, polyacrylonitrile,polytetrafluoroethylene, polyhexafluoropropylene, polymethylmethacrilateand the like are used alone or by having two or more thereof formed as acopolymer.

Next, a description is given to the present invention according to thespecific exemplary embodiments but the present invention is not limitedin whatever manner by these exemplary embodiments.

Table 1 shows the structures of the first to fifth exemplary embodimentsusing the electrode materials of the present invention and also priorart examples 1 and 2 as control examples. Table 2 shows capacitance perunit weight and the rate of change of capacitance after 500 hours at 85°C. for the first to fifth exemplary embodiments of the present inventionand the prior art examples 1 and 2.

TABLE 1 Electrolyte/ Solvent Electrode Material First ExemplaryTEAClO₄/PC Polyethylenedioxithiophene/ Embodiment polystyrene copolymerSecond Exem- TEMAPF₆/PC Poly 3-(4-fluorophenyl) thio- plaryphene/polyethylene oxide Embodiment copolymer Third Exem- EMIBF₄/PC Poly(5,8-diaminoanthraquinone)/ plary polyvinylidene fluoride copolymerEmbodiment Fourth Exem- TEMAClO₄/AN Polyaniline/polyacrylonitrile plarycopolymer Embodiment Fifth Exemplary TEMAIPNS/ANPolypyrrole/polypropylene oxde Embodiment copolymer + manganese dioxidePrior Art TEABF₄/PC Activated carbon Example 1 Prior Art TEAClO₄/PC3-(4-fluorophenyl) thiophene Example 2 TEACl₄; Perchloric acidtetraethylammonium, TEMAPF₆; Phosphorous triethylmethylammoniumhexaflouride, EMIBFphd 4; Boric acid 1 - ethyl - 3 methylimidazoliumquadflouride, TEMAClO₄; Perchloric acid triethylmethylammonium,TEMAIPNS; triethylmethylammonium tri-isopropylnaphtalenesulfonate,TEABF₄; Boric acid tetraethylammonium quadflouride, PC; Propylenecarbonate, AN; Acetonitrile

TABLE 2 Capacitance Rate of Change of (F/g) Capacitance (%) FirstExemplary Embodiment 35 −15 Second Exemplary Embodiment 50 −17 ThirdExemplary Embodiment 52 −20 Fourth Exemplary Embodiment 39 −15 FifthExemplary Embodiment 42 −13 Prior Art Example 1 20 −10 Prior Art Example2 60 −97

As clearly seen in Table 2, the electrochemical capacitors in the firstto fifth exemplary embodiments of the present invention have largecapacitance values per unit weight in comparison with theelectrochemical capacitor in the prior art example 1 and show excellentstability at high temperatures when compared with the electrochemicalcapacitor in the prior art example 2.

INDUSTRIAL APPLICABILITY

As described in above, an electrochemical capacitor of the presentinvention is allowed to have electrode materials enhanced in heatresistance and cycle characteristics by having the copolymerizationbetween a conductive polymer and a polymer with excellent heatresistance and a powerful affinity for a solvent taken place on theelectrode of the electrochemical capacitor, thereby enabling therealization of an electrochemical capacitor with a higher energy densitythan that of a prior art electric double layer capacitor using activatedcarbon as the electrode thereof and further with more enhanced heatresistance and cycle characteristics than those of an electrochemicalcapacitor using a conductive polymer as the electrode thereof.

What is claimed is:
 1. An electrochemical capacitor comprising a pair ofoxidation-reduction active electrodes with an electrolyte or an ionicconductive solid electrolyte sandwiched between said oxidation-reductionactive electrodes, wherein said oxidation-reduction active electrodecontains a copolymer expressed by a chemical formula 1; wherein thechemical formula 1 is shown as: R^(a)—(C₂R^(c) ₄)_(l)—(OC₂R^(d)₄)_(m)—(X)_(n)—R^(b), or R^(a)—[—(C₂R^(c) ₄)_(l)—(OC₂R^(d)₄)_(m)—(X)_(n)—]_(p)—R^(b)  (Chemical Formula 1) , where R^(a) and R^(b)are, respectively, hydrogen, fluorine, an alkyl group with 1 to 15 ofcarbon, a fluoroalkyl group, an alkenyl group and a fluoroalkenyl group,or a phenyl group, a fluorophenyl group, a sulfone group, an aminogroup, a nitro group and a cyano group, and R^(a) and R^(b) are allowedto be the same with or different from each other; and R^(c) and R^(d)are selected from hydrogen, fluorine, an alkyl group with 1 to 15 ofcarbon, a fluoroalkyl group, an alkenyl group and a fluoroalkenyl group,or a phenyl group, a fluorophenyl group, a sulfone group, an aminogroup, a nitro group and a cyano group, or a chemical formula A, andsaid respective four R^(c) and R^(d) are allowed to be the same with ordifferent from one another; and the chemical formula A is described by ageneral expression as follows: —(CO)OR^(e), —(CH₂)R^(a)—(OC₂R^(b)₄)_(b)—OR^(e) , where R^(e) is hydrogen or an alkyl group, and “a” and“b” are integers of 0 to 20 and allowed to be the same with or differentfrom each other; in the chemical formula 1, “l” and “m” are integers of0 or larger and not 0 simultaneously, and “n” and “p” are naturalnumbers of 1 or larger; X is any one species, or two or more species outof chemical species described by a general expression as follows:

R1 to R6 are, respectively, hydrogen, fluorine, an alkyl group with 1 to20 of carbon, which is allowed to be replaced with a hydroxyl group, afluoroalkyl group, an alkenyl group and a fluoroalkenyl group, or aphenyl group, a fluorophenyl group, a sulfon group, an amino group, anitro group and a cyano group with part or all thereof being allowed tobe linked with one another to form rings; and R1 to R6 are allowed to bethe same with or different from one another.
 2. The electrochemicalcapacitor according to claim 1, wherein X in the chemical formula 1 is3-(4-fluorophenyl) thiophene.
 3. The electrochemical capacitor accordingto claim 1, wherein X in the chemical formula 1 is 3-sulfonylthiophene.4. The electrochemical capacitor according to claim 1, wherein X in thechemical formula 1 is 3,4-disulfonylthiophene.
 5. The electrochemicalcapacitor according to claim 1, wherein X in the chemical formula 1 isethylenedioxithiophene.
 6. The electrochemical capacitor according toclaim 1, wherein said oxidation-reduction electrode contains at leastone or more selected from a group of oxides of ruthenium, manganese,vanadium, titanium, aluminum, nickel, iron, magnesium and silicon.